Syrian media say the attack outside Damascus killed two people. Western reports, however, say the strike hit a convoy carrying arms for Hezbollah in Lebanon.

JERUSALEM — Israeli warplanes struck targets outside the Syrian capital, Damascus, on Wednesday, according to Syrian and Western reports, amid rising international fear that President Bashar Assad could lose control of his nation's stockpiles of chemical and advanced weapons.

A Western official who spoke on condition of anonymity confirmed that the airstrike hit a truck convoy believed to be carrying antiaircraft weapons for Hezbollah militants in neighboring Lebanon. The shipment was thought to have included Russian SA-17 missiles, the official said. If such weapons were obtained by the Islamic militant group, it could weaken Israel's regional military power and hinder its ability to launch airstrikes in Lebanon.

Syrian state media, while also reporting an Israeli airstrike, denied that the target was a weapons shipment for Hezbollah, instead claiming that a military research facility and adjacent building had been destroyed. It said two people were killed and five were injured in the dawn attack.

Syria did not say what kind of research took place at the facility in Jamraya, northwest of the capital.

Israeli officials declined to comment on the reports. But such a strike would mark Israel's most aggressive military action in Syria during the nearly two-year uprising against Assad's rule.

Israeli officials have been sounding alarms in recent days that Syria's weapons might fall into the hands of militant groups that could use them against Israel. Prime Minister Benjamin Netanyahu raised those concerns during a Cabinet meeting this week and officials have repeatedly said that any transfer of Syria's dangerous weapons outside the country might trigger a military response.

A Syrian military statement carried by the official Syrian Arab News Agency charged that the attack proved that Israel was behind efforts to oust Assad. "It has become clear to everyone that Israel is the motivator, beneficiary and sometimes executor of the terrorist acts which target Syria and its resistant people," it said.

Israel has tried to steer clear of the Syrian conflict, fearing any actions it might take, such as supporting opposition forces or launching a military strike, could backfire or become a propaganda coup for Damascus. Syrian officials have long charged that U.S. and "Zionist" forces are behind the rebellion against Assad. Each side in the Syrian conflict has portrayed itself as an implacable enemy of Israel.

There are also fears that an Israeli strike could draw others into the Syrian conflict. Iran, Syria's close ally, said this week that any foreign attack against Syria would be regarded as an attack on Iran.

In addition to chemical weapons, Israeli officials have been particularly worried about Syria's stockpile of SA-17 antiaircraft missiles.

"The initial speculation was about chemical weapons, but Israel is deeply concerned about Hezbollah acquiring this kind of advanced antiaircraft missile," said Jonathan Spyer, a senior research fellow at the Center for Global Research in International Affairs in Herzliya, Israel. "It would transform Hezbollah's game and potentially end Israel's air superiority over Lebanon. This is entirely about Hezbollah, not about Syria."

Officials in Lebanon denied knowledge of any Israeli attack, though they complained that Israeli jets had violated their airspace beginning Tuesday afternoon and through Wednesday morning. Though Israel routinely flies over Lebanon for reconnaissance missions on Hezbollah arms stocks and movements of weapons, the activity overnight was reportedly heavier than usual.

Israel often refuses to confirm or deny its activities in the region, partly out of a belief its silence might reduce the pressure on its enemies to respond.

Israeli is widely believed to have struck a Syrian nuclear facility in 2007. But it has never acknowledged the attack and Assad's regime, which was eager to avoid a direct confrontation with Israel, also dropped the sensitive matter.

In recent weeks, Israelis warned that Assad is losing control over his chemical weapons and military action might be taken.

"If we have solid evidence, shared by our own partners all over the world, that chemical warheads are being transferred from Syria to Lebanon to Hezbollah, no one will condemn Israel for trying to prevent it," said retired Brig. Gen. Amnon Sofrin, a defense contractor and former senior intelligence officer.

Amid the renewed warnings, Israelis living in the northern part of the country near the borders with Syria and Lebanon have been swarming into post offices and other distribution centers to pick up government-issued gas masks.

Israel relocated two of its five Iron Dome missile-interception systems to the northern part of the country, though military officials said the move was not related to fears of impending attacks.